1. Field of the Invention
The present invention relates to a display drive device for driving a display unit having an electrochromic element and a display device provided with the same.
2. Description of the Related Art
There have been proposed various display devices having a display unit provided with a display element (namely, an electrochromic element) using a phenomenon such that a solute in an electrolyte separates out or dissolve due to an oxidation-deoxidization reaction by injecting an electric charge in the electrolyte. For example, in the case of the display device using bismuth (Bi) as a main material, when a negative electric charge is injected from a transparent electrode such as an indium tin oxide (ITO), a Bi ion in the electrolyte is deoxidized to separate out on the transparent electrode and this part is colored black. On the contrary, when a positive charge is injected from the transparent electrode into the electrolyte, Bi which has separated out on the transparent electrode is oxidized to solve into the electrolyte and this part is decolored.
In this type of display device, the amount of the solute to separate out is determined depending on the amount of the electric charge injected into the electrolyte, so that the larger the area of the display unit is, the more the amount of the solute should be increased by injecting more electric charges into the electrolyte.
In order to inject the electric charge according to the area of the display unit in this way, conventionally, the electric charge is injected by supplying a constant voltage for a predetermined time. In the case of driving the display unit by supplying the constant voltage from the display drive device for driving the display unit to the display unit, it has been known that a resistance value of the display unit at the time of coloration is in inverse proportion to the display area if a wiring resistance up to the display unit can be ignored. In addition, it has been known that the resistance value is in inverse proportion to the display area also at the time of decoloration until the decoloration is completed, as well as at the time of coloration. As a result, when the constant voltage is supplied from the display drive device for a predetermined time, it is possible to inject the electric charge of the amount which is approximately in proportion to the display area.
However, a method for simply supplying a predetermined amount of voltage waveform involves a problem that it is difficult to keep a balance between the amount of the solute to separate out and the amount of the solute to be dissolved, and an operating life is short. In other words, if coloration and decoloration are repeated many times, the balance between the amount of the solute to separate out and the amount of the solute to be dissolved is deteriorated, and this easily causes a defect where a part of the colored portion does not decolor due to excessive deposition and the electrolyte may change colors due to generation of other substance (for example, an iodine) in the electrolyte because dissolution of the solute has been progressed too much. Particularly, if time from a coloration step to a decoloration step is made longer, the decoloration may naturally occur and this easily generates excessive decoloration.
In the case of decoloration, differently from coloration, when the decoloration has progressed to some degrees, it has been known that the resistance value is changed and a current is decreased. In order to compensate decrease of the amount of the electric charge due to decrease of the current, the voltage in the case of the decoloration should be higher than that in the case of coloration or the voltage supplying time should be longer, because the amount of the electric charge necessary for decoloration is equivalent to that necessary for coloration. In this case, a voltage drop is generated due to a wiring resistance between the display unit and the display drive device and a contact resistance between the circuits or the like and the flowing current is largely changed, so that it is necessary to control the supplied voltage and the supplying time in consideration of these effects. However, the value of the wire resistance and the value of the contact resistance between the circuits generally vary widely and it is not easy to make these values constant.